That is "the aim of the ecumenical movement [that] remains unchanged" says Papa Benedict. I think that this is at the core of how we must understand the Lord's fervent prayer "that they may be one" in John 17. In the few conversations I've had with Protestants, I get the feeling that they'll only go so far, reluctantly, as to concede that visible unity is a good thing, but they'll be one step short and say that the Lord surely did not mean that it would happen on earth. It isn't possible, after all, given how frail we are.
What a lack of confidence in the God who saves, and a betrayal of the mission of the Church, and her establishment by Christ to provide the unbelieving world with a sign, a city built on a hill, a means to sanctify the whole world.
And so pride wins. Or perhaps despair. Or perhaps a lack of faith. Perhaps all three, but it usually gives me the impression that we just don't seem to have enough trust. Trust the Holy Spirit! We must acknowledge that what is impossible for us is very possible for God. After all, we who have been reborn of water and Spirit are no longer offspring of sin but offspring of holiness.